1. Field of the Invention
The present invention relates to a method and an apparatus of detecting coordinates in a gas discharge panel.
2. Description of the Prior Art
Conventionally, the dot matrix type display device, called the plasma display panel, is known in the art as a display device utilizing gas discharge. The conventional plasma display panel is described in various publications, for example, U.S. Pat. No. 3,499,167, entitled "Gas Discharge Display Memory Device and Method of Operating", by Baker et al, and U.S. Pat. No. 3,559,190, entitled "Gaseous Display and Memory Apparatus", by Bitzer et al. Such a plasma display panel comprises a gaseous cell filled with gas capable of ionization, and arrays of X and Y electrodes arranged and connected in matrix form so as to provide many "elemental discharge areas" in the gaseous cell. Further the major characteristic feature is to provide a memory function due to wall charges. However, in spite of this characteristic feature the circuit for driving this plasma display panel requires one address circuit for each X and Y electrode. As these address circuits are required to scan in order, the peripheral circuit of this plasma display panel becomes very complex and expensive. In this connection, the conventional plasma display panel is somewhat disadvantageous compared with other devices such as cathode ray tube display devices.
Another display device utilizing gas discharge provides a self scanning device as a means for resolving the above-mentioned inconveniences of the peripheral circuit. The trademark name of the device is "Self Scan". This kind of display device as mentioned, for example, in ELECTRONICS (in U.S.A.), Vol. 43, No. 5, P120-P125, comprises double construction of the scanning part and display part. Discharge sports are moved along each line of the scanning part on the rear face used as an exciting source. The discharge spots are scanned and selectively taken to the display part on the front face, in accordance with the position of said discharge spot, and thereby the display action is carried out.
However, the above-mentioned type of the display device utilizing gas discharge has some other disadvantages. That is, the construction of a wall for separating the scanning part and the display part, and the construction of the electrodes, is complicated and not suited to mass production. Also the life of each electrode exposed directly to the gaseous cell is short, and an external memory for refreshing is necessary. Further the brightness of the display part decreases as the number of picture elements increases.
Still another display device, which is called a "self shift type plasma display panel", is disclosed in U.S. Pat. No. 3,944,875, entitled "Gas Discharge Device Having a Function of Shifting Discharge Spots", by OWAKI et al. Such a self shift type plasma display panel comprises a gaseous cell filed with gas capable of ionization, at least one row of electrodes, and a plurality of column electrodes positioned in such a manner that they cross the row of electrodes. In this self shift type plasma display panel, the elemental discharge areas are arranged sufficiently close together that adjacent elemental discharge areas affect each other in the direction of the shift of the discharge spot by the primary current effect.
As used herein, the term "elemental discharge area" means the minimum area which can be fired in the gaseous cell. The minimum area is positioned between the cross points of row and column electrodes, or between adjacent electrodes. The term "primary current effect" means an excitation generated in one elemental discharge area by firing an adjacent one.
The self shift type plasma display panel mentioned above has a construction in which a plurality of electrodes crossing the direction of shifting of the discharge spot are connected periodically to common buses and each common bus is respectively connected to a sustaining signal source having a predetermined phase. In this gas discharge panel, a voltage higher than the discharge initiating voltage is applied between the row electrodes and the column electrodes to generate discharge spots, and the discharge spots on the display device can be shifted by using the primary current effect. Consequently a desired display pattern having discharge spots can be obtained.
Display patterns generated in the above mentioned self shift plasma display panel can be used for various displays. When this gas discharge display is used, for example as a display of a computer system, it is necessary to use a man-machine interface enabling an operator to have conversations with the computer through the gas discharge panel. In order to realize this man-machine interface, it is first of all indispensable that information of the coordinate position on the gas discharge panel designated by the operator should be converted to some electrical signal.
A selected letter or item displayed on the gas discharge panel can be identified by this coordinate position information. In an ordinary display device, a light pen is used for designation of the detected point, and a method of detecting scanned light spots is customarily adopted. When this conventional means is applied to the gas discharge panel, the intended object can be attained by moving a scanning discharge spot along a predetermined route, along a large number of discharge cells arranged with high density in a matrix pattern, by obtaining information of the coordinates (X, Y) as an electric signal at the position where the light pen detects this scanning discharge spot. In this case, however, disadvantages are caused. For example, since the scanning discharge spot is superposed on the display pattern already being shown, this is caused to disappear and erroneous lighting takes place.